This invention relates generally to pulse forming apparatus, and more particularly to a pulsed power modulator circuit for a repetitively pulsed chemical oxygen-iodine laser in which the threshold of the resonator is reduced by the incorporation therein of a scaleable intracavity gas phase Q-switch.
It is well recognized in the art that photochemical iodine lasers are capable of generating pulsed emission of very short duration and high energy. Examples of such photochemical iodine lasers can be found in the "Handbook of Chemical Lasers," edited by R. W. F. Gross and J. F. Bott, John Wiley and Sons, New York, 1976, chapter 12, pages 670-701. In the photochemical iodine laser the population inversion is produced in the flash photolysis of a parent alkyl-iodide, generally C.sub.3 F.sub.7 -I or CF.sub.3 -I. The iodine atom that is produced in the photolysis process is in the upper laser level, .sup.2 P.sub.1/2.
High energy pulsed lasers of this type require large stores of electrical energy and generally operate at efficiencies of less than a few percent. In recent years a new type of iodine laser, the chemical oxygen iodine laser, COIL, has been under development. In this laser the population inversion is produced by energy transfer from excited molecular oxygen in the O.sub.2 (.sup.1 .DELTA.) state. In the most recent development stages the emphasis has been placed on supersonic chemical oxygen iodine lasers. In the supersonic mode, thermally induced medium effects and their influence on beam quality are much smaller by comparison with subsonic operation. In subsonic operation of the COIL under loaded cavity conditions power extraction with its attendant heat release occurs so rapidly that unacceptable density variations are produced in the flow direction. Supersonic operation of COIL lasers, however, requires the generation and transport of O.sub.2 (.sup.1 .DELTA.) at high pressure (&gt;10 torr) which is difficult.
It is therefore highly desirable to be able to effectively utilize the low pressure generator technology and subsonic flow in chemical oxygen-iodine lasers in which the aerodynamic problems associated with supersonic flow would be much less critical.
U.S. patent application Ser. No. 785,186 entitled "REPETITIVELY PULSED Q-SWITCHED CHEMICAL OXYGEN-IODINE LASER" discloses an invention which overcomes the problems of severe flux induced density gradients in a continuous wave subsonic cavity of a chemical oxygen-iodine laser by operating the laser in a repetitively pulsed mode through the incorporation therein of a scaleable intracavity gas phase Q-switch. In this repetitively pulsed Q-switched chemical laser, the extraction volume defined by the resonator of the chemical oxygen-iodine laser is filled during the interpulse time with a thermally uniform gain medium. Once the optical mode volume is filled, laser action is triggered by reducing the threshold of the resonator with a scaleable intracavity gas phase Q-switch. For subsonic flow, cavity volumetric exchange times are on the order of milliseconds, whereas pulse extraction times due to the large magnitude of the O.sub.2 (.sup.1 .DELTA.)/I transfer rate are on the order of microseconds. During the laser pulse, the medium is essentially stationary with temporal density variations caused by the flux induced temperature rise occuring uniformly over the optical aperture. Thus, by operating the laser at subsonic velocity in a repetitively pulsed mode the single most critical issue, that of medium quality and its effect on beam quality, is substantially reduced. In addition, the short duration, high intensity pulses produced in this particular mode of operation offers significant advantages in terms of propagation, target interaction effects, and the potential for frequency doubling when compared with lower average power CW operation.
The peak power enhancements in the repetitively pulsed mode of operation of the Q-switched chemical oxygen-iodine laser come about as a direct result of the relatively long radiative and collisional lifetimes of the .sup.2 P.sub.1/2 state which permits and efficient accumulation of energy within the resonant cavity during the switch off portion of the cycle. The particular intracavity gas phase switch arrangement of this invention is based upon the application of the Zeeman effect.
The aforementioned U.S. patent application Ser. No. 785,186 discloses several alternative embodiments utilizing the repetitively pulsed Q-switch mode of operation. In each of these concepts the resonant cavity input flow is continuous and an iodine atom absorption region is placed intracavity and colinear with the optical axis of the laser. The absorption regions are configured with solenoids, i.e, coils of wire that produce magnetic fields therein which are parallel to the optical axis of the laser. When the magnetic field is off the absorption region counteracts the laser region and oscillation does not occur. In all of these concepts O.sub.2 (.sup.1 .DELTA.), i.e., oxygen, and I.sub.2, iodine, are supplied continuously to the cavity region.
In the preferred embodiment of the Q-switched chemical laser disclosed in U.S. patent application Ser. No. 785,186, the absorbing iodine atoms are produced in a separate chemical oxygen-iodine generator. This absorption region is coupled directly to the chemical oxygen-iodine gain region. Repetitive pulse operation occurs by first allowing the gain region to fill with fresh media when the field is off. Once the extraction volume is filled, a fast rising current pulse is applied to the solenoids producing transparency in the absorption region. With the gain medium essentially stationary, a short duration high intensity pulse is then extracted from the gain region. The peak intensity and pulse width are determined by the concentrations of O.sub.2 (.sup.1 .DELTA.) and iodine atoms.
In an alternate embodiment of the Q-switched laser disclosed in U.S. patent application Ser. No. 785,186, the iodine atoms are produced in a heated cell from thermal dissociation of I.sub.2. The heated cell is configured with a solenoid that produces an axial magnetic field. The heated cell is then repetitively pulsed modulating the absorption and producing a train of laser pulses in the manner set forth above. In a further alternate embodiment of the Q-switched laser disclosed in U.S. patent application Ser. No. 785,186, the natural build-up and decay of the subsonic gain profile is utilized. The gain region is folded within the resonator through the absorption region. In the absorption region the subsonic flow passes through the solenoid field coils. When the gain is positive in the upstream section the absorption in the downstream section is reduced by applying a current pulse to the solenoid. Net gain is established in the resonator and the energy in the upstream section of the flow is extracted in a short duration laser pulse.
In each of the embodiments of a Q-switched laser discussed above, it is necessary to provide a pulsed power modulator circuit which is capable of providing pulses of high current and preferably having fast rise times and fall times through the magnetic field-forming coils or solenoids.